The present disclosure relates to a fixing device which fixes a toner image on a sheet and an image forming apparatus including the fixing device.
An electrophotographic type image forming apparatus, such as a copying machine and a printer, is provided with a fixing device which fixes a toner image transferred on a sheet on the sheet. The fixing device includes a fixing member to be heated and a pressing member being pressed against the fixing member to form a pressing area. By making the sheet pass through the pressing area, the toner image is heated and pressed and then fixed on the sheet.
The fixing member includes a thin tube and a coating layer provided around the thin tube. The thin tube is made of aluminum or iron, and the coating layer is made of PFA or PTFE. The coating layer eliminates an adhesion of the toner on the fixing member and charging of the fixing member. In order to prevent occurrence of wrinkle on the sheet, the fixing member may be formed in an inverted crown shape in which an outer diameter is smaller at the center portion than at the both end portions in a sheet width direction perpendicular to a sheet conveying direction. The fixing device further includes a separating claw which comes into contact with the fixing member with a predetermined pressure and separates the sheet from the fixing member. The separating claw is made of polyimide resin coated with PFA, for example.
In order to reduce energy consumption of the fixing device with recent energy saving, a low melting point toner and a fixing member having a low heat capacity are developed. As a specific method to make the fixing member have a low heat capacity, the fixing member is made to have a small diameter or a thin thickness. Additionally, as a heater for heating the fixing member, an inexpensive halogen heater is conventionally used. Corresponding to a high-speed image forming apparatus, a plural of heaters may be used. The plural of heaters include a main heater heating the center portion in the sheet with direction and a sub heater heating the both end portions in the sheet width direction. In view of fixing ability at continuous printing, a power consumption of the main heater is made to be higher than that of the sub heater.
In a case where the plural of heaters are used, because rush current increases if they are turned on at the same time, the heaters are turned on with time difference. Conventionally, the sub heater is firstly turned on and then the main heater is turned on after one cycle of a lighting time of the heater (for example, 200 ms in case of 50 Hz) elapses. However, if the heaters are turned on at such a timing, because the main heater has a power consumption larger than that of the sub heater, a period in which a temperature of the center portion rises to a fixing allowable temperature is shorter than a period in which a temperature of the both end portions rises to the fixing allowable temperature. Then, until the temperature of the both end portions rises to the fixing allowable temperature and the fixing member is uniformly heated in the sheet width direction, it is required to wait the start of the fixing operation. During the waiting time, in order to keep the temperature of the center portion the fixing allowable temperature, the main heater is turned on and off at a constant interval, and unnecessary power is consumed by the turn on/off of the main heater during the waiting time.
The fixing device is sometimes configured to turn on a heat source having a smaller power consumption first among a plural of heat sources. Alternatively, the fixing device is sometimes configured to include a first heater having a small rush current and a second heater having a large rush current and fast temperature rising time, and to turn on the first heater first.
In the fixing devices having the above configurations, it becomes possible to eliminate the increasing of the rush current. However, they have no consideration about the temperature unevenness due to a difference in the temperature rising period between the center portion and the both end portions and about increasing of the power consumption.